Device and method for compression moulding plastic containers

ABSTRACT

A compression-moulding press for moulding plastic containers (P) consisting of a moulding cavity formed by a plunger ( 5 ), a female mould ( 14 ), and two separable lips ( 7, 8 ). A cylinder ( 16 ) slides inside a sliding slide ( 22 ) to keep the lips ( 7, 8 ) closed. The plunger ( 5 ) slides inside the cylinder ( 16 ), with respect to both the cylinder ( 16 ) and the slide ( 22 ), when moulding the containers (P). Finally, a method for making plastic containers by means of compression moulding using said device is also described.

FIELD OF THE INVENTION

This invention relates to a device and a method for compression mouldingof plastic items. More particularly, it is especially suitable formoulding parisons for making plastic bottles, pots, containers, orsimilar items through blow moulding.

PRIOR ART

The parisons for making blow-moulded containers, especially made of PET,are normally made by means of injection, compression, or extrusionmoulding.

Adopting compression moulding for the production of parisons has thefollowing advantages over injection moulding:

-   -   Lower moulding temperature and pressure, and, thus, a lower        level of residual acetaldehyde in the finished        bottle—acetaldehyde also contributes to the unpleasant apple        taste found, for example, in bottles of mineral water stored in        hot warehouses for a long time.    -   Less degradation of the plastic material due to the breakage of        its polymeric chains.    -   Less mechanical stress due to the lower moulding pressure, and,        thus, a smaller press.    -   Elimination of injection-point defects on the finished parison        that cause quality problems and the explosion of bottles in the        next blow-moulding operation.    -   Easier pairing of the moulding station with the rest of the        plant, which generally includes machines that operate        continuously.    -   Elimination of the storing areas.    -   Filling the different moulding cavities with more uniform doses        and at more uniform temperatures.

Normally, during compression moulding, a predetermined amount of plasticmaterial, in fluid or pasty form, is deposited in the cavity of a mould.Then, the mould is closed, and the plastic is shaped by pressing it asdesired. Differently from injection moulding, after closing the mould,the moulding cavity does not communicate—for example, with aplasticization screw—with the external environment, rather it remainsisolated without exchanging plastic material or air; normally, thismeans that the moulding cavity is filled with a dose that is slightlyless that the maximum theoretical amount. This, together with thecontraction in volume of the plastic due to cooling in the mouldingcavity, makes it difficult to obtain moulded pieces with particularlyprecise sizing tolerances.

This makes it difficult to compression mould objects such as parisonsfor processing later through blow moulding, which must have quiteprecise sizing tolerances and shapes; in fact, filling errors, bubbles,and other geometric irregularities can lead to explosions in theblow-moulding phase. Furthermore, the shrinkage of the plastic materialin the neck and threading areas of the parison can lead to unacceptableappearance problems and problems when screwing the bottle cap.

In accordance with a Swiss patent, CH 664 526 (Maegerle), said problemsare tackled when compression-moulding plastic tubes for packagingtoothpaste with a press where the moulding plunger is kept underpressure during the cooling phase of the plastic. In this way, theplunger continues its stroke compensating for the thermal shrinkage ofthe plastic material, which is constantly compressed against the wallsof the moulding cavity; the finished workpieces have a better sizingprecision.

On the downside, taking into account the shrinkage of the materialduring compression by increasing the pressure of the plunger makes itharder to remove the object after cooling. This results in a ratherwidespread problem where, when compression-moulding objects such asparisons for blow-moulding containers, it is necessary to use notableforce in order to extract the plunger, which shapes the internal cavityof the parisons, from the parison after cooling.

Various compression-moulding operations are known for making relativelysmall plastic objects such as caps for bottles, but these solutions arenot immediately applicable for making large or deep plastic containersthat generally require high outputs.

OBJECTS OF THE INVENTION

It is an object of this invention to provide a device and a method forcompression-moulding plastic items that resolves the aforementionedproblems, especially with regard to the force required to release themoulded workpiece at the end of the moulding cycle and the removal ofthe workpiece from the moulding station.

In accordance with a first aspect of the invention, these objects areachieved by means of a device for compression-moulding plasticcontainers, having an internal cavity communicating with the outsidethrough an opening, featuring, in accordance with claim 1, a mouldcomprising a plunger to shape at least part of said internal cavity ofthe container and a female mould to shape part of the external surfaceof the container. Said plunger and female mould are able to reach acoupling position in order to shape the workpiece by sliding in theopening or closing direction of the mould. The assembly also features atleast two moving completion elements to finish said external surface ofthe moulding cavity. The assembly also includes a slide, which slides inthe opening or closing direction of the mould, equipped with acylindrical cavity and a cylinder able to slide inside the cylindricalcavity in the opening or closing direction of the mould and to keep themould in the closed position by applying a force on said completionelements; furthermore, the plunger can slide along the opening orclosing direction of the mould inside the cylinder for at least asegment of its length with respect to said cylinder and said slide.

In accordance with another aspect of the invention, these objects areachieved by means of a method for moulding plastic containers,implemented by means of a device described above, which in accordancewith claim 9, comprising the following stages:

-   -   a) Filling a cavity of a female mould with a dose of fluid        plastic;    -   b) Feeding the slide with the cylinder and the plunger toward        said female mould;    -   c) Closing the completion elements between said female mould and        said cylinder;    -   d) Closing the mould and compression moulding the container by        advancing the plunger inside the female mould;    -   e) Applying an appropriate continuous compression force on the        plunger in order to further sink the plunger into the plastic,        depending on the contraction in volume, in order to compensate        for thermal shrinkage during cooling;    -   f) Detaching the plunger from the cavity of the female mould        making it slide with a first stroke along said mould-opening        and/or closing direction while said cylinder and said completion        elements remain in the closed-mould position.

The fact of having a moulding unit with a sliding plunger, at least inthe mould-opening and closing direction, with respect to the compressioncylinder and the sliding slide, makes it possible and more convenient touse a gas spring system in order to operate the compression cylinder. Infact, the gas spring system is simpler, less expensive, and has fasterresponse times compared to a system using, for example, an oleo orpneumatic system to operate a compression cylinder, while still beingable to apply high compression forces. Furthermore, said gas springmakes it possible to unload the moulded workpiece, and in particular toremove the plunger from even a deep-moulded object, at faster speedsthan not having a plunger essentially integral with the sliding slide.

According to the above method, the plunger is detached from the mouldedobject—which requires the greatest force in the mould-openingphase—using the slide actuators. The slide actuators are also used tokeep closed the other parts of the mould during the detachment of theplunger: in this way, it is possible to avoid useless oversizing of theremoving actuators. Finally, the mould and its moving members—forexample, the lips and the bar of the lips—are stressed optimally andwear better.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the invention will be readily apparent from the moredetailed description of a particular version of the invention, given asa nonlimiting example and in conjunction with the following accompanyingdrawings:

FIGS. 1 through 8 show a schematic view of the vertical sections of apreferred version of a device for compression-moulding plastic objectsin accordance with this invention during some phases of a mouldingoperation;

FIG. 9 shows a perspective view of a detail of the device shown in FIG.1;

FIG. 10 shows a side view of the detail shown in FIG. 9;

FIG. 11 shows a section view along the B-B plane of the detail shown inFIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, the terms “continuously operated press”and “continuously operated moulding unit” are used to refer to a devicewhere the moulding cycle is carried out in several moulding cavitieswith a timing difference between the different cavities: for example,while a moulding cavity is opened to be filled, another unit is closingto compress, and a third unit is opened to remove the moulded workpiece.The terms “sequential press” and “sequential moulding unit” refer to amoulding device where the moulding cycle is executed simultaneously andwithout timing differences between the different moulding cavities: forexample, all the moulding cavities are filled, or closed to compress, oropened to remove the moulded workpieces.

FIG. 1 shows a mould 1 suitable for compression mould of plastic objectscomprising a female mould 14 where there is a moulding cavity 2 fordepositing some plastic, for example, PET, PP, PS, PE, PVC, PEN, PBT,etc., in fluid form or in a consistency suitable for moulding.

The female mould is fixed onto the bottom mould-holding plate 3 of amoulding unit 4 in accordance with this invention. Several units 4similar or identical to the one shown in FIGS. 1 through 8 are fittedonto a support that rotates around a vertical axis in order to create arotary carousel for compression moulding plastic objects that, accordingto the preferred version of this invention, are parisons suitable to betransformed, with a subsequent blow-moulding operation, into bottles formineral water, drinks, or fruit juices, pots for example, for storingpreserved food, or other types of containers.

The mould 1 also includes an essentially cylindrical plunger 5 in alengthened shape suitable to define the shape of the internal cavity 6of the parison P, and two completion elements 7, 8, shown more clearlyin detail in FIGS. 9 through 11, which will be called “lips”hereinafter. These lips are used to shape the known type of mouth andneck of the parison: in particular, the outside of the threaded end ofthe container—the round protrusion called “ring” and the roundprotrusion called “sealing ring” on which any tear sealing rings of thebottle cap are fastened. In order to be able to extract the parison fromthe mould, the lips 7, 8 are separated from each other by amould-dividing surface in the form of a vertical surface, and open andclose together with a horizontal movement, or anyhow transversal to theapproach stroke of the plunger to the female mould. The direction ofsaid stroke will be called “mould-opening and/or closing direction”hereinafter, meaning the main mould-opening/closing direction, referringto the approach direction of the larger elements of the mould.

The scope of this invention also covers completion devices, instead ofthe lips 7, 8, that have an equivalent function and can be detached fromthe moulded object P with a movement at least transversal to the openingand/or closing direction of the mould.

In accordance with the preferred version of the invention shown in theattached Figures, the lips 7, 8 open and touch each other rotatingaround a vertical bar 12, FIG. 11, creating a type of scissor-likemechanism.

The shape of the lips 7, 8 is suitable to create, when closed, a hole 13within which the plunger 5 can be fed with a sliding movement.

When the mould is closed FIGS. 3 through 6, the two lips touch eachother and rest on the female mould 14, and the plunger 5 is fit into thehole 13.

Preferably the vertical bar 12, which acts as hinge for the lips, isplaced in the part of the unit 4 facing the inside of the rotarycarousel, while the parts of the lips that define the hole 13 are in aposition that is more external with respect to the bar 12: thisarrangement makes it easier to reach the core of the mould 1 withfilling devices and devices for removing the moulded parisons, and isparticularly appropriate for moulding stations consisting of manymoulding units 4 operating continuously, rather than sequentially.

Ideally, the plunger 5 is fastened to, or is machined from, a bar 15that serves to move the plunger.

The lips 7, 8 are blocked axially onto the bar 12, and can be raised andlowered by making the bar 12 slide according to the opening or closingdirection of the mould.

Preferably, during moulding, a hollow cylindrical element, or hollowpiston 16—hereinafter referred to as “hollow presser” 16 FIG. 1—slidesvertically and compresses the lips 7, 8 closed against the female mould14, preventing the mould 1 from opening due to the internal mouldingpressure. To accomplish this, the hollow presser 16 and the top end ofthe female mould 14 ideally both have a housing in the shape similar toa truncated cone 34 and 36, respectively, or anyhow operating accordingto the principle of an inclined plane, apt to couple with the shape of atruncated cone 35 and 35A, respectively of the closed lips 7, 8 so thatthe compression force exerted by the hollow presser 16 and by the femalemould 14 tends to keep the lips closed.

Alternatively, the lips can be more than two and kept closed with thecompression cylinder 16, or with other appropriate pressing means notnecessarily hollow able to keep the mould 1 closed by applying acompressing or closing force at least on the lips.

The sliding slide 22 consists of a support apt to slide at leastaccording to the opening and closing direction of the mould. In theexample shown in the Figures, said slide is positioned above the femalemould 14 and the bottom mould-holding plate 3 and can go up or down—forexample, thanks to a cam-type operation, not shown—with a preferablyvertical stroke, while—as shown in the example—the bottom mould-holdingplate 3 is at a fixed height and can only rotate around a vertical axisdriven by the moulding carousel.

According to a first aspect of this invention, the plunger 5 and theplunger-holding bar 15 can slide along the mould-opening and/or closingdirection with respect to the hollow presser or cylinder 16 and withrespect to the slide 22.

The bar 15 of the plunger 5 slides in the cylindrical internal cavity ofthe hollow presser 16 with a movement independent from the movement ofthe latter, as is described more in detail below.

Preferably, the horizontal opening and closing movement of the lips 7, 8is controlled by a cam-type device 17—better shown in FIGS. 1, 9, 10,and 11. Two wheels 18, 19, each rotating around a pin integral with thelips 7, 8, slide along two slits 20 and 21, respectively that define thesliding shape of the cam.

Preferably, the sliding housings of the bars 12, 15 and of the hollowpresser 16 are machined from a single slide 22 FIG. 1. Preferably, thetwo slits 20, 21 of the cam, which controls the opening of the lips, aremachined from a plate 25, for example made of steel FIG. 11, fastenedrigidly to the slide 22.

The sliding between the bar 12 of the lips and the slide 22 iscontrolled by a cam-800 operation and by the cylindrical chamber 23,also machined from the slide 22.

The cam-type operation consists of a tappet, or follower, for example ofthe wheel type, fastened onto the bar 12 of the lips that follows,during the rotation of the moulding carousel, the cam 800 shape, whichis fixed with respect to the frame of the carousel.

The cylindrical chamber 23 is filled with an essentially fixed amount ofpressurized gas, for example air or nitrogen, at an indicative pressureof 40 bars, in order to behave like a gas spring that tends to push thebar 12 of the lips constantly upward. The operating principle of the cam800 shape will be explained below.

Analogously, the sliding between the bar 15 for operating the plungerand the slide 22 is controlled by a cam-900 operation and by thecylindrical chamber 24, also machined from the slide 22 and filled withan essentially fixed amount of pressurized gas, for example air ornitrogen, at the indicative pressure of 40 bars. Preferably, the bar 15that operates the plunger 5 features a section with a larger diameter26, FIG. 6 above and below which there are two segments 27, 28 of thebar diameter of Dsup and Dinf, respectively. The section with the largerdiameter 26 connects to the two upper 27 and lower 28 segments of thebar with two flat ring-type surfaces 29 and 30, respectively orthogonalto the axis of the bar 15. Preferably, Dsup is greater than Dinf so thatthe force of the pressurized gas in the chamber 24 on the surfaces 29and 30 tends constantly to push the bar 15 upward against the top end ofthe cylindrical chamber 24 or of the cam 900.

The wall of the cylindrical chamber 24 and the hollow presser orcylinder 16 that slides inside are formed in order to create, with thehelp of appropriate sealing, airtight sliding. Similar airtight slidingis created between the internal cavity of the hollow presser 16 and theplunger-holding bar 15, which slides inside. This minimizes the lossesthrough seeping of the pressurized gas trapped inside the cylindricalchamber 24, and the hollow presser 16 and cylindrical chamber 24assembly behaves like a second gas spring that tends constantly to pushthe hollow presser downward against the bottom of the second cylindricalchamber 24.

The use of gas springs 23, 24, and 25 offers different advantagesincluding, for example:

-   -   The possibility of making many gas springs, two for each        compression unit 4, with more contained stiffness tolerances and        with less decay due to fatigue compared to, for example, steel        springs    -   Simplicity and cost effectiveness of making the springs compared        to a system of hydraulic cylinders piloted by valves    -   The possibility of handling with a very simple device, and with        relatively fast response times, relatively high pressures and        compression forces, for example 40 bars on a cylinder of        approximately 160 mm of diameter produce a force of more than        8×104 N

Preferably, the top surface or shoulder 29 and the top wall 31 of thechamber 24 are made so that they can abut mechanically against eachother and so that they can push the plunger 5 with the force of theslide 22 during the compression and mould-closing phase.

The bar 15 for operating the plunger comes out of the top part of theslide 22—which has an external contact surface 37 machined, for example,from a sliding sleeve 38—on which a corresponding contact surface 39,machined from the bar 15, comes into mechanical contact when lowered.

The slide 22 and the plunger-holding rod 15 are made so that the slide,by means of the contact surfaces 37, 39, can push the plunger 5 and theplunger-holding rod 15 when the mould 1 opens.

FIGS. 1, 8, and 8A show a schematic view of part of an external devicefor removing parisons moulded by the moulding unit 4. The removingdevice features a star wheel 32 around which a concentric guide 33 isplaced.

What follows is a description of the operation of a moulding unit 4.

FIG. 1 shows a moment in the moulding cycle when the cavity 2 of thefemale mould 14 has just been filled with a predetermined dose of meltedplastic, for example PET.

The lips 7, 8 are in their position of maximum horizontal opening andare located slightly above the star wheel 32 and the removing guide 33;the operating bar 15 of the plunger 5 is also raised so that its bottomend is above the star wheel 32 and the removing guide 33.

The slide 22 is positioned at a height so that its surface 29 abutsmechanically against the top wall 31 of the chamber 24.

The pressure of the air in the chamber 24 keeps the hollow presser 16pressed against the bottom of the chamber 24 and in its position ofmaximum extension outside said chamber.

FIG. 2 shows a subsequent moment of the moulding cycle when, aftermaking the bar 12 of the lips slide with respect to the plate 25 of thecams, the lips 7, 8 are closed and are brought—thanks to the gas spring23 and the cam 800—into contact with the hollow presser 16. The slide 22of the press is being lowered with respect to the mould-holding plate 3,the pressure of the gas inside the chamber 24 continues to keep theshoulder 29 of the bar 15 of the plunger in contact with the top wall 31of the chamber, and the hollow presser 16 is in its position of maximumstroke extended outside the chamber 24; thus, the bar 15 of the plungerand the hollow presser 16 follow rigidly the lowering movement of theslide 22.

FIG. 3 shows the moment when the mould 1 closes, the moulding cavity ofthe mould 1 is isolated from the external environment, and the lips 7, 8come into contact with the top edge of the female mould 14 and arecompressed by the hollow presser 16; also the horizontal opening of thelips 7, 8 is prevented by coupling the surfaces of the lips in the shapeof a truncated cone 35, 35A with the corresponding surfaces of thehollow presser 16 and of the female mould 14 in the shape of a truncatedcone 34, 36. The lips 7, 8 remain in this status also in the momentsshown in FIGS. 4, 5, 6.

Returning to FIG. 3, the mould is preferably closed before the plunger 5starts to immerse itself in the fluid plastic in order to preventsplashing or similar when the mould is still open.

Until now the slide 22, the hollow presser or cylinder 16, the bar 15 ofthe plunger, and the lips 7, 8 have been lowered independently from eachother.

FIG. 4 shows a next moment when the hollow presser 16 remains immobilecompared to the bottom mould 14 and continues to press against the lips,blocking them; the slide 22 continues to lower itself and, since theshoulder 29 is in contact with the top wall 31 of the chamber 24, pushesthe bar 15 of the plunger downward; the plunger 5 has been immersed inthe fluid plastic making it occupy the entire moulding cavity andexpelling, through appropriate blowholes, all the air that was inside.

The blowholes, and the sliding opening between the plunger 5 and thehole 13 of the closed lips, are of a size and thickness suitable to letair pass but trap the fluid plastic.

When the air has been totally expelled, there is a definite increase inthe resistance to let the plunger 5 lower further. The lowering of theslide 22 and, consequently, of the plunger 5 when this is immersed inthe fluid plastic is controlled by means of a logical unit andappropriate sensors or equivalent control devices of the mechanical,pneumatic, or other type. This serves to apply on the plunger 5 apushing force never greater than the maximum force, which once exceededimmediately stops or slows down the lowering of the plunger 5,continuing anyhow to apply on the melted plastic said maximum force;this is equal to compressing the plastic in the mould with a pressurelimited to a maximum value. Said maximum push value is chosen in orderto make the air come out completely from the closed moulding cavitywhile preventing the melted plastic from being extruded through theblowholes.

In the next moments, between the one of FIG. 4 and the one of FIG. 5,the plunger 5 continues to apply on the plastic an appropriatecompression force in order to make the plunger 5 sink further into theplastic at a slower rate. This rate depends on the contraction in volumeof the plastic due to its cooling; the goal being to push the plasticagainst the walls of the moulding cavity to compensate for the thermalshrinkage due to the cooling.

This sinking, in the case of parisons of the type shown in FIG. 1A, canbe indicatively of 3 to 7 mm depending also on the dimensions of theparison. During sinking, the bar 15 of the plunger continues to bepushed downward by the slide 22 since the top shoulder 29 remains incontact against the wall 31 of the chamber 24 of the slide. The hollowpresser 16 continues to enter into the chamber 24, pressing against thelips that rest in turn on the female mould 14.

In FIGS. 4 and 5, the chamber 40—formed by the bottom shoulder of thehollow presser 16, its sides, and the side walls of the chamber24—communicates with the external atmosphere.

This phase is stopped, for example, after a predetermined time, or whenthe estimated temperature of the plastic material in the mould issufficiently low.

With dimensional tolerances of the different elements of the mould andthe dose of the plastic deposited in the mould sufficiently precise, itis possible to achieve, when the sinking of the plunger 5 is stoppedFIG. 5 and the moulded plastic object is removed from the mould, aworkpiece with the desired shape and dimensional tolerances.

The next FIGS. 6 to 8 refer to the operations of opening the mould andremoving the moulded object.

When the plastic in the mould is sufficiently cool, the plunger 5 is nolonger sunk into the mould. Now, the plunger 5 is detached from insidethe parison P: the hollow presser 16 continues to press the lips 7, 8downward and against the female mould 14, the slide 22 is lifted and thebar 15 of the plunger remains still and blocked for the time being withrespect to the parison P. When the contact surface 37 of the tubularsleeve 38 abuts against the contact surfaces 39 of the bar 15 of theplunger, the latter is raised with significant force by the slide 22;the plunger 5 is detached from the internal cavity 6 of the parison PFIG. 6, which is firmly held by the lips 7, 8. Preferably, this firstraising of the plunger 5 from the cavity 6 of the parison is stoppedafter a few millimetres in order to have a first limited stroke, as willbe clarified below.

Carrying out the initial detachment of the plunger 5 from the parisonwith the driving force of the slide 22 and the abutment 37, makes itpossible to size the Dsup and Dinf diameters in the segments 27, 28 ofthe bar 15 in order to have relatively small push values of the gas inthe chamber 24 on the plunger-holding bar 15. Said values are such thatthey do stress excessively the cam 900 but are sometimes insufficient todetach with the only hydrostatic affect the plunger 5 from the parisonP. Said release force can be, for example, even of 400 daN.

Then, the lips 7, 8 are lifted following the upward movement of theslide 22 and of the bar 15 of the plunger so that there is no slidingbetween the parison P, the plunger 5, and the lips 7, 8.

In order to do this the bar 15 of the plunger continues to abut againstthe tubular sleeve 38, the hollow presser 16 continues to press againstthe bottom wall of the chamber 24, and the bar of the lips 12 is pushedupward and kept abutting against the hollow presser 16 by thepressurized gas in the chamber 23. The cam 800 acts as safety if thispush is inadequate to lift the lips; the cam 900 prevents theplunger-holding rod 15 from rising too fast and too much, hindering theupward push on the rod 15 given by the pressurized gas in thecylindrical chamber 24.

The three sliding elements 12, 15, 16 rise together with the slide 22and the parison is completely removed from the female mould 14.

The bar 12 of the lips stops its upward movement FIG. 7 when the parisonP is located at an appropriate/predetermined height in order to be readyto be gripped by a removing device, which includes the removing star 32and the removing guide 33. Ideally, the lips 7, 8 do not changesubstantially said position in height during the next phases.

Then, FIG. 7, the bar 12 of the lips and the cams on the plate 25 slideslightly one compared to the other in order to open the lips 7, 8slightly.

The plunger 5, already detached from the cavity 6 of the parison, isstill almost completely inside said cavity. When the two lips 7, 8 areopened, the plunger 5 constrains the horizontal movements of the parisonP in order to detach the threading and ring areas of the parison fromboth lips, instead of just from one lip. The opening of the lips in thisphase is limited so that these still hold the ring of the parison and donot allow it to fall.

The cam 900 contrasts the upward push of the gas in the chamber 24 onthe plunger-holding bar 15, keeping the abutment 29 detached from thetop back wall of the chamber 24.

FIG. 8 shows a next moment of the release of the parison: the rotationof the moulding carousel moves the parison P—held by the lips 7, 8almost at the same height of FIG. 7—in the field of action of theremoving device 32, 33.

With an adequately fast movement—controlled by the cam 900 and helped bythe upward push of the gas of the cylindrical chamber 24—the plunger 5is removed completely from the parison P; then, the lips 7, 8 arecompletely opened releasing the parison, which can be grasped by thestar-removing device 32, 33, removed by the moulding unit 4 andtransferred to the transport or processing stations after the mouldingcarousel.

In the version shown in FIGS. 7-8, the length H of the cylindricalchamber 24 is insufficient to enable the complete removal of the plunger5 from the parison P exclusively making the surface 29 of theplunger-holding bar abut against the top back wall of the chamber 24;thus, it is necessary to help the extraction of the plunger 5 by liftingthe slide 22 from the position shown in FIG. 7 to the one in FIG. 8.

At the end of the moulding cycle, in FIG. 8, the different movingelements of the moulding unit 4 are found in the same positions as inFIG. 1, ready to carry out a new compression cycle.

Carrying out the detachment of the plunger 5 from the internal cavity 6of the parison as first release operation, and in particular when thelips 7, 8 are still solidly closed by the hollow presser 16 preventssticking between the plunger 5 and the internal walls of the lips; thus,the coupling between the lips/plunger 5 can be realized with very smalldiameter tolerances.

Delaying the complete removal of the plunger 5 until almost the end ofthe moulding cycle makes it possible to detach the parison from the lips7, 8 preventing this from sticking; and it is possible to unload theparison from a rotating moulding machine with many moulding units 4operating continuously.

The fact of positioning the lips 7, 8, since the phase shown in FIG. 7,at the height at which the parison P will be gripped by a removingdevice such as the star wheel 32 facilitates and speeds up the unloadingof the moulded parison. For example, this is a better solution comparedto the system FIG. 10 of letting the moulded parison fall by opening thelips 7, 8 in a predetermined unloading and removing position.Furthermore, this solution allows the parisons to be removed from themould at a higher temperature and when the plastic material has not yethardened completely.

Having the plunger 5 fastened on a plunger-holding bar 15 sliding bothwith respect to the hollow presser 16 and the slide 22, makes itpossible firstly to extract the plunger faster from the parison P whenremoving it with the removing star 32 or other removing devices, in FIG.8. Making the plunger 5 and the plunger-holding bar 15 withappropriately reduced weight and inertia makes it easier to liftquickly—at least for a segment of the stroke—only the plunger-holdingbar 15 instead of the entire slide 22.

Secondly, this makes it possible to operate the hollow presser orcylinder 16 with a simple gas spring 24—or with a system of steelsprings or other elastic similar elements—without having to resort to ahydraulic or pneumatic cylinder and relating supply and control circuit.Obviously, this makes it significantly simpler and cost effective torealize numerous compression units 4, which are still able to operatethe hollow presser 16 with high compression forces.

The version described above can undergo several changes and still remainwithin the scope of this invention.

For example, the internal cavity 6 of the parison P or other hollowobject can be shaped with multiple plungers 5—meaning a plunger 5consisting of several moving elements, instead of a single plunger 5;the lips 7, 8 can be blocked and kept closed, during the furtherlowering of the plunger P, with fastening devices fasted onto the bottommould-holding plate 3 instead of onto the top slide 22; similarly, thesections 20, 21 of the cam for controlling the opening and closing ofthe lips 7, 8, or the sliding housings 23 of the bar for lifting thelips can be fastened onto a different support from the slide 22, forexample, onto the bottom mould-holding plate 3 instead of on the slide22 or other; the lips 7, 8 or other elements that open with a movementtransversal to the main opening direction of the mould in order toenable the removal of undercut parts can be more or less than two, orcan be formed by one or more elements that make it possible, forexample, to remove a threaded moulded part with an unscrewing rotationparallel to the axis of the thread; and, finally, the mechanicaloperating parts can be replaced with electric, oleo, or other type ofcontrols.

1. A device for compression moulding plastic containers (P), having aninternal cavity (6) communicating with the outside through an openingcomprising a mould (1) with a plunger (5) to shape at least part of saidinternal cavity (6) of the container (P), a female mould (14) to shapepart of the external surface of the container (P), being able to reach areciprocal coupling position in order to shape the container (P) bysliding in the opening or closing direction of the mould (1), and atleast two moving mould completion elements (7, 8) to finish saidexternal surface of the moulding cavity, said device also including aslide (22), being able to slide in the opening or closing direction ofthe mould, the slide being provided with a cylindrical cavity (24) and acylinder (16) able to slide inside the cylindrical cavity (24) in theopening or closing direction of the mould and to keep the mould (1) inthe closed position by applying a retaining force on said mouldcompletion elements (7, 8), and characterised in that it comprises meansfor operating the plunger so that said plunger (5) can slide along theopening or closing direction of the mould (1) inside the cylinder (16)for at least a segment of its length with respect to said cylinder (16)and said slide (22), and means for applying an appropriate continuouscompression force on said plunger (5) in order to further sink saidplunger into the plastic, depending on the contraction in volume, inorder to compensate for thermal shrinkage during cooling.
 2. A device asclaimed in claim 1, wherein said completion elements (7, 8) move bymoving away transversally to said opening and closing direction of themould in order to detach from the moulded container (P).
 3. A device asclaimed in claim 2 wherein the slide (22) and the plunger (5) featurerelating first contact surfaces (29, 31) apt to abut against each otherduring the mould-closing and compression operations in order to slidesolidarity.
 4. A device as claimed in claim 3 wherein the slide (22) andthe plunger (5) feature relating second contact surfaces (37, 39) areadapted to abut against each other reciprocally during the mould-openingoperation in order to slide solidarity.
 5. A device as claimed in claim4 where said cylinder (16) comprises a portion shaped as piston thatslides inside said cylindrical chamber (24) containing gas in order tocreate a gas spring.
 6. A device as claimed in claim 5 wherein theplunger (5) in the segment found inside said chamber (24) features twosegments of its length of a different diameter.
 7. A device as claimedin claim 6 wherein said completion elements (7, 8) are able to slidealong a direction parallel to the opening and closing direction of themould in order to move nearer and further away from the female mould(14) and/or from the slide (22) during specific phases of the mouldingoperation.
 8. A device as claimed in claim 7 wherein said opening andclosing and sliding movements of the completion elements (7, 8) arecontrolled by a cam-type device fastened to the sliding slide (22).
 9. Amethod for moulding plastic containers (P), implemented by means of adevice as claimed in one of the previous claims, comprising thefollowing stages: Filling a cavity of a female mould (14) with a dose offluid plastic; Feeding the slide (22) with the cylinder (16) and theplunger (5) toward said female mould (14); Closing the completionelements (7, 8) between said mould and said cylinder (16); Closing themould (1) and compression moulding the container (P) by advancing theplunger (5) inside the female mould (14); Applying an appropriatecontinuous compression force on the plunger (5) in order to further sinksaid plunger into the plastic, depending on the contraction in volume,in order to compensate for thermal shrinkage during cooling; Detachingthe plunger (5) from the cavity of the female mould (14) making it slidewith a first stroke along said mould-opening and/or closing directionwhile said cylinder (16) and said completion elements (7, 8) remain inthe closed-mould position;
 10. A method as claimed in claim 9 whereinthe container (P) is completely removed from the plunger (5) by makingthe plunger, and the completion elements (7, 8) carry out reciprocalmoving away movements.
 11. A method as claimed in claim 10 wherein saidcompletion elements (7, 8) are opened in order to release the grip onthe container (P), and allow the container to be gripped by removingmeans (32, 33).